Nuclear brominated phenolic compounds, such as tetrabromobisphenol A (TBBA) and tribromophenol, are used in plastics materials and synthetic resins as flameproofing agents. TBBA is particularly important for this purpose, since it can be incorporated reactively into polymer systems, for instance epoxy resins and polyesters, via the two phenolic hydroxyl groups. For this application, it is necessary to use high quality tetrabromo-4,4'-alkylidenediphenols; the products must be as free as possible from byproducts, have as low as possible a color number, and be essentially free from hydrolyzable bromine.
Commercial grades of TBBA sometimes have high APHA color values, so that their use, particularly in transparent products, leads to undesirable colorations. Other difficulties arise if the content of aliphatically combined bromine is high, because these substances can cause corrosion through the hydrolytic or thermal elimination of hydrobromic acid and reduce the electrical insulation values of synthetic resins flameproofed with such TBBA. In many cases the plastics processor is forced to purify standard grades himself or to use special grades, each of which leads to increased costs.
The bromination of 4,4'-alkylidenediphenols with bromine to the nuclear brominated tetrabromo-4,4'-alkylidenediphenols is well known. The hydrobromic acid liberated during the bromination can be oxidized to bromine and re-used. Sulphur trioxide and chlorine have been used for the reoxidation. However, their use leads to considerable environmental pollution because of the formation of sulphur dioxide or hydrogen chloride.
A simple and environmently acceptable process, by which phenols and 4,4'-alkylidenediphenols are brominated in presence of hydrogen peroxide, has been described in the U.S. Pat. No. 3,929,907. In this process, which also is used in the present invention, the hydrobromic acid arising in the bromination is oxidized in situ by aqueous hydrogen peroxide solution to bromine, which is then available for further bromination. The peroxy-bromination of 4,4'-alkylidenediphenols, such as bisphenol A, takes place at 0.degree. C. to 100.degree. C. in the presence of an inert organic solvent which is not miscible with water but is suitable for the crystallization of the tetrabromo-4,4'-alkylidenediphenols. After the bromination, the reaction mixture is heated sufficiently to dissolve the suspended reaction product in the organic phase, and the aqueous phase can be separated; then the organic phase is cooled, causing the brominated product to crystallize following which the product can be isolated in known manner.
By multiple reuse of the mother liquor of the process of U.S. Pat. No. 3,929,907, the yield can indeed be increased, but it has been found that this reduced the quality of the tetrabromo-4,4'-alkylidenediphenol. This becomes particularly clear in the APHA color number and the saponifiable bromine content, which approximately doubles even after a single recycling of the mother liquor. In the previously known peroxybromination of bisphenol A to TBB carried out on the industrial scale using chlorobenzene as solvent, without mother liquor recycle, the TBBA has an APHA color number of about 150 and a saponifiable bromide content of about 600 ppm; this quality could only be maintained in subsequent batches if the mother liquor was first freed from quality-lowering substances by redistillation before being reused. A redistillation of the entire mother liquor is very expensive.